1. Field of the Invention
This invention relates to a corrosion resistant PC steel stranded cable for use with a prestressed concrete structure and also to a process of and an apparatus for producing such corrosion resistant PC steel stranded cable.
2. Description of the Prior Art
Conventionally, a high strength steel stranded cable is widely used as a PC steel element for applying prestress to concrete of a prestressed concrete structure in a pretensioning method or a posttensioning method.
A corrosion resistant PC steel stranded cable is already disclosed, for example, in Japanese Patent Laid-Open No. 130960/1984, Japanese Patent Publication No. 47609/1978 or Japanese Utility Model Laid-Open No. 144121/1986 which is obtained by applying suitable corrosion resisting processing to a high strength steel stranded cable.
Referring to FIG. 17, there is shown a corrosion resistant PC steel stranded cable which is disclosed in Japanese Patent Laid-Open No. 130960/1984. The corrosion resistance PC steel stranded cable 10F shown includes a bare PC steel stranded cable 10 composed of a plurality of side wires 10a and a core wire 10b, and a coating layer 11 of paint of a synthetic resin material such as an epoxy resin coated on an entire outer periphery of the bare PC steel stranded cable 10.
In the corrosion resistant PC steel stranded cable 10F, a plurality of spiral air gaps 12 are defined between the side wires 10a and the core wire 10b and extend along a longitudinal direction of the corrosion resistant PC steel stranded cable 10F while a spiral recessed groove 13 is defined on the outside of and between each adjacent ones of the side wires 10a within the coating layer 11 and extends in the longitudinal direction of the corrosion resistant PC steel stranded cable 10F. In order to fill up such spiral recessed grooves 13 completely, it is necessary to partially increase the thickness of the synthetic resin paint coating layer 11, and if the synthetic resin paint coating layer 11 is so formed, then the thickness of a plurality of spiral recessed grooves 14 which are formed on an outer face of the synthetic resin paint coating layer 11 and extend in a longitudinal direction of the coating layer 11 will be decreased as much. In other words, the size of the spiral recessed grooves 14 is decreased.
Such corrosion resistant PC steel stranded cable 10F is adopted in a pretensioning method and imparts tensile force applied in advance thereto to concrete due to sticking thereof to the concrete. However, the corrosion resistant PC steel stranded cable 10F is disadvantageous in that such sticking force decreases as the size of the spiral recessed grooves 14 decreases and consequently introduction of prestress thereof into the concrete becomes insufficient.
Further, since the PC steel stranded cable 10F has the spiral air gaps 12 therein, if the opposite longitudinal end faces are not enclosed perfectly, air or water may be admitted into and flow in the air gaps 12 and cause corrosion of the wires 10a and 10b.
In addition, if the synthetic resin paint coating layer 11 should have a pin hole or should be damaged upon placing of concrete, then air or water may be admitted into the spiral air gaps 12 through the pin hole or damage of the synthetic resin paint coating layer 11 and may cause internal corrosion of the PC steel stranded cable 10F.
FIG. 18 shows a corrosion resistant PC steel stranded cable disclosed in Japanese Patent Publication No. 47609/1978. Referring to FIG. 18, the corrosion resistance PC steel stranded cable 10G shown includes a bare PC steel stranded cable 10 having a similar construction to that of the bare PC steel stranded cable 10 shown in FIG. 17. Lubricant 15 of the oil type which serves also as an anticorrosive is applied to a surface of the bare PC steel stranded cable 10, and a coating tube 16 of paint of a synthetic resin material is fitted around the bare PC steel stranded cable 10.
The corrosion resistant PC steel stranded cable 10G is adopted in a posttensioning method and, after concrete is hardened, the corrosion resistant PC steel stranded cable 10G is strained and fixed to the concrete to provide prestress to the concrete. When such prestress is to be introduced, the sticking force to the concrete need not be called in question, but on the contrary the bare PC steel stranded cable 10 must necessarily be moved in its longitudinal direction with respect to the concrete.
The corrosion resistant PC steel stranded cable 10G is disadvantageous in that the lubricant 15 is deteriorated as time passes or the applied condition of the lubricant 15 likely becomes imperfect. It is another disadvantage that, if the synthetic resin paint coating tube 16 is damaged by some causes upon placing of concrete or the like, then the corrosion resisting property is deteriorated extremely.
A corrosion resistant PC steel stranded cable which eliminates such drawbacks of the corrosion resistant PC steel stranded cable 10G has been proposed and is shown in FIG. 21. Referring to FIG. 21, the improved corrosion resistant PC steel stranded cable 10J shown is similar in construction to the corrosion resistant PC steel stranded cable 10F shown in FIG. 17 in that an entire outer periphery of a bare PC steel stranded cable 10 is covered with a coating layer 17 of paint of a synthetic resin material such as an epoxy resin and a coating tube 18 of paint of a synthetic resin material is fitted around the synthetic resin paint coating layer 17 on the corrosion resistant PC steel stranded cable 10F by protrusion molding.
However, the improved corrosion resistant PC steel stranded cable 10J is still disadvantageous in that, since a plurality of spiral air gaps 12 are formed between a plurality of side wires 10a and a core wire 10b similarly as in the corrosion resistant PC steel stranded cable 10F of FIG. 17, air or water is likely admitted into the spiral air gaps 12 by way of openings of the longitudinal opposite end faces of the corrosion resistant PC steel stranded cable 10J or a pin hole or a damaged portion of the synthetic resin paint coating layer 17.
Meanwhile, Japanese Utility Model Laid-Open No. 114121/1986 discloses two different corrosion resistant PC steel stranded cables. One of the two corrosion resistant PC steel stranded cables is shown in FIG. 19. Referring to FIG. 19, the corrosion resistant PC steel stranded cable 10H shown includes a coated PC steel stranded cable 10' composed of a plurality of side wires 10a and a core wire 10b which are each coated with a coating layer 22 of paint of a synthetic resin material and are stranded with one another. Lubricant 19 is applied to a surface of the coated PC steel stranded cable 10', and a coating tube 20 of paint of a synthetic resin material is fitted around the coated PC steel stranded cable 10'.
FIG. 20 shows the other corrosion resistant PC steel stranded cable disclosed in Japanese Utility Model Laid-Open No. 114121/1986. Referring to FIG. 20, the corrosion resistant PC steel stranded cable 10I shown includes a coated PC steel stranded cable 10' of the similar construction to that of the corrosion resistant PC steel stranded cable 10H, and a coating layer 21 of paint of a synthetic resin material is provided directly around an outer periphery of the coated PC steel stranded cable 10' without applying lubricant to the coated PC steel stranded cable 10'.
The corrosion resistant PC steel stranded cables 10H and 10I shown in FIGS. 19 and 20 are superior in corrosion resisting property because the side wires 10a and the core wire 10b are individually covered with the separate synthetic resin paint coating layers 22.
However, to this end, such synthetic resin paint coating layers 22 must naturally be formed in advance on the side wires 10a and the core wire 10b before they are stranded with each other. However, due to presence of the synthetic resin paint coating layers 22, heat treatment necessary to remove residual stress after stranding operation, that is, blueing, cannot be performed.
Blueing is performed such that, after the stranding operation of the bare PC steel stranded cable 10, it is heated to about 400.degree. C. to remove residual stress after working to stabilize the stranded condition of the bare PC steel stranded cable 10. If such blueing is not performed, then a problem will arise that, each time the bare PC steel stranded cable is cut into a segment of a required length, the strand or twist of the bare PC steel stranded cable will be loosened or cancelled from a thus cut end portion of the bare PC steel stranded cable.
Even if it is tried to conduct suitable working in order to maintain the strand or twist without performing blueing, this is accompanied by extreme difficulty, and besides, upon such working, there is the possibility that the synthetic resin paint coating layer 22 may be damaged. Thus, it is a disadvantage that an anticipated corrosion resisting effect cannot be maintained.